Causes and consequences of variation in offspring body mass: meta‐analyses in birds and mammals

Abstract

Early survival is highly variable and strongly influences observed population growth rates in most vertebrate populations. One of the major potential drivers of survival variation among juveniles is body mass. Heavy juveniles are better fed and have greater body reserves, and are thus assumed to survive better than light individuals. In spite of this, some studies have failed to detect an influence of body mass on offspring survival, questioning whether offspring body mass does indeed consistently influence juvenile survival, or whether this occurs in particular species/environments. Furthermore, the causes for variation in offspring mass are poorly understood, although maternal mass has often been reported to play a crucial role. To understand why offspring differ in body mass, and how this influences juvenile survival, we performed phylogenetically corrected meta-analyses of both the relationship between offspring body mass and offspring survival in birds and mammals and the relationship between maternal mass and offspring mass in mammals. We found strong support for an overall positive effect of offspring body mass on survival, with a more pronounced influence in mammals than in birds. An increase of one standard deviation of body mass increased the odds of offspring survival by 71% in mammals and by 44% in birds. A cost of being too fat in birds in terms of flight performance might explain why body mass is a less reliable predictor of offspring survival in birds. We then looked for moderators explaining the among-study differences reported in the intensity of this relationship. Surprisingly, sex did not influence the intensity of the offspring mass-survival relationship and phylogeny only accounted for a small proportion of observed variation in the intensity of that relationship. Among the potential factors that might affect the relationship between mass and survival in juveniles, only environmental conditions was influential in mammals. Offspring survival was most strongly influenced by body mass in captive populations and wild populations in the absence of predation. We also found support for the expected positive effect of maternal mass on offspring mass in mammals (r = 0.387). As body mass is a strong predictor of early survival, we expected heavier mothers to allocate more to their offspring, leading them to be heavier and so to have a higher survival. However, none of the potential factors we tested for variation in the maternal mass-offspring mass relationship had a detectable influence. Further studies should focus on linking these two relationships to determine whether a strong effect of offspring size on early survival is associated with a high correlation coefficient between maternal mass and offspring mass.

Drivers and demographic consequences of seasonal mass changes in an alpine ungulate

“…A recent meta-analysis showed that maternal mass provides a reliable predictor of offspring birth mass in mammals (Ronget et al, 2018). However, heavier females in autumn had a higher probability of lactation the following year (GLMM, b = 1.25, 95% CI [0.84, 1.71]) and their lambs were more likely to survive the neonatal period (GLMM, b = 0.32, 95% CI [0.15, 0.48]).…”

“…Lamb mortality is approximately three times greater in winter than in summer (Portier et al 1998). Because heavy female mammals generally produce heavy offspring that show high survival rates (Ronget et al, 2018;see Clutton-Brock et al 1996, Griffin et al 2011 for case studies), we expected that lamb survival should increase with mid-September maternal mass. Because females that lost more mass during winter must gain more mass during the following summer (Pelletier et al 2007b), and because mothers favor their own summer mass gain over the growth of their lambs (Festa-Bianchet and Jorgenson 1998), we predicted that lamb summer mass gain should be lower for mothers that lost relatively more mass during the previous winter.…”

Thermal constraints on body size depend on the population’s position within the species’ thermal range in temperate songbirds

“…In France, the impact of warming on avian population dynamics may impact body size negatively at the warmest sites, corresponding to the Mediterranean region, where species (including our study species) live closer to the upper boundary of their thermal tolerance (Jiguet et al, ). As individual fitness is often size‐dependent, with the largest surviving and/or reproducing better (Lindström, ; Ronget et al, ), it is to be feared that size decline may impact populations negatively. To assess the robustness and generality of our conclusions, future studies should investigate the synergistic effects of local temperature anomalies (weather fluctuations) and local average temperatures (climate) on body size across a larger number of species (including a broader spectrum of life‐history traits, and particularly more thermal specialists), and across a broader climatic range, to include the entire thermal ranges of the species studied.…”

Colony‐specific differences in decadal longitudinal body composition of a capital‐breeding marine top predator

“…Festa-Bianchet, Gaillard, & Jorgenson, 1998;Ronget et al, 2018;Schick et al, 2013;Sheldon & West, 2004). Festa-Bianchet, Gaillard, & Jorgenson, 1998;Ronget et al, 2018;Schick et al, 2013;Sheldon & West, 2004).…”

“…Resource limitation forces long-lived iteroparous animals to trade off survival, growth, and reproduction during their lifetime (e.g. Festa-Bianchet, Gaillard, & Jorgenson, 1998;Ronget et al, 2018;Schick et al, 2013;Sheldon & West, 2004). Body composition provides a measure of the immediate phenotypic quality of individuals within a population and can serve as a rough proxy for their relative fitness-animals in 'good' condition can fulfil their own energetic needs and store spare resources for future use, potentially deriving fitness benefits over the long term (Speakman, 2001).…”